A balun is a circuit component frequently used to connect together sections of different impedances. A balun is essentially a transformer connecting first and second circuits that generates a signal in a first circuit based on the signal in a second circuit. More particularly, a balun can convert a signal that is symmetrical (or “balanced”) about ground to a signal that is asymmetrical (or “unbalanced”) about ground and vice versa.
In a transceiver, transmitting and receiving circuitry are commonly arranged to receive a differential input from a balun whilst the antenna is arranged on the unbalanced side. This means that the choice of balun and of the various components used in the two types of circuitry is made in dependence on each other. When choosing a balun for a particular circuit, an engineer will make a compromise between these two design constraints.
For example, a power amplifier (PA) in the transmitting circuitry ideally requires a balun that supports large currents and which has a high quality factor, Q, for a high efficiency and PA linearity. The quality factor is a measure of how quickly energy is dissipated from an oscillating system. A high Q means that energy is lost slowly whilst a low Q means that energy is lost quickly. In contrast to the transmitting circuitry, the low noise amplifier (LNA) in the receiving circuitry ideally requires a high voltage gain with a low noise figure (NF). As NF is directly related to the Q of the balun, it can be seen that the choice of balun is a compromise between these two opposing design considerations.
With both the transmitting and receiving circuitry on the one side, additional components need to be added to isolate components in the transmitting circuitry from components in the receiving circuitry. If the circuits were not isolated from each other during operation, damage could occur to these components. This possibility of damage restricts the choice of possible LNA core devices and topology. For example, sometimes a “slow” MOS device with a long channel length is selected for the LNA in the receiving circuitry. This choice can reduce the damage caused to the LNA by the large switching voltages used by the PA in the transmitting circuit. However, slow MOS devices are not preferred for LNA's that need to operate at high frequencies.
There are two primary ways in which the two circuits are isolated from each other: Firstly, the PA and LNA may be shared on the balanced side of the balun, as illustrated in FIG. 1. This configuration may include switching units and/or additional baluns; Secondly an external switch may be used, as illustrated in FIG. 2.